The spine is a complex structure of vertebral bones and connective tissues. The vertebral bodies, discs and ligaments are configured in operative engagement to perform flexibility for motion, support for body forces and protection for the spinal cord. The structure and functionality of this complex arrangement can be compromised if one or more components are diseased or injured. Often it is possible to treat these problems with conservative non surgical methods such as physical therapy, medication and or exercise. In other instances, such as degenerative instability, deformity, trauma, tumors, etc., necessitate surgical intervention to treat unstable vertebral joints and treat pain caused by root compression.
There are a variety of surgical procedures that result in fixation of portions of the spine to one another. In most instances fixation of the spine involves the use of bone screws. These screws are implanted through surgical openings in adjacent portions of the spine and threaded into the spine members. The bone screws are then mechanically connected through additional components such as rods and plates.
A number of internal spinal fixation systems have been used in spinal surgery to achieve a rigid fixation. A common technique is the use of posterior implants using pedicle screws coupled to longitudinal rods. Typically posterior implants include a one or more pairs of rods that are positioned along the longitudinal axis of the spine. These rods are attached to the spinal column by bone screws that are threaded into the pedicles of the individual vertebral bodies. The procedure for posterior fixation includes the steps of insertion of the screws into the pedicles in a particular angle and depth, temporarily coupling the rods to the screws, correction of the spinal curve, and the securing of the connection rods to the screws for rigid fixation. The alignment of the pedicle screws is predicated upon the angle and depth of screw insertion which is dependent by the patient's anatomical and pathological conditions that may vary at different spinal levels as well as between patients. A lack of proper alignment will cause considerable difficulty during this procedure. Attempts to compensate for this lack of alignment can result in the creation of undesirable forces being applied to the spinal column via the pedicle screws and will extend the time needed to perform the operation, thereby increasing the possibility of complication associated with surgery. While the surgeon can form a bend in the rod or plate to ensure that the screws are firmly secured to the plate or the rod, such a modification may create stresses that may result in the vertebrae to fracture or the screws to loosen over a period of time. Therefore is highly desirable that the bone fixation system minimizes the possibility of creating unnecessary excessive loads on the spinal column.